1. Field of the Invention
This invention relates to a method of manufacturing readily machinable high strength glass ceramics from which a product excelling in electrical insulation performance, heat resistance and mechanical strength and providing a precise size accuracy can be obtained with ease through machining. More specifically, this invention relates to a method of manufacturing readily machinable high strength glass ceramics which are valuable for use as materials for the heat-resistant insulating parts (e.g., substrates, carriers, etc.), the structure support members (e.g., coil bobbins, sensor supports, etc.), etc. of an electric/electronic apparatus, a precision mechanical apparatus, etc.
2. Description of the Related Art
Examples of known readily machinable ceramics include: the ceramic disclosed in Japanese Patent Laid-Open No. 63-50365 that is composed of mica powder and a ZnO-SiO.sub.2 -B.sub.2 O.sub.3 -type ceramic, the ceramic disclosed in Japanese Patent Publication No. 64-345 that is composed of mica powder and cordierite, and the ceramic obtained by precipitating mica crystals in glass, as shown in "Glass Handbook" (by Sumio Sakubana, Teruo Kyohno and Katsuaki Takahashi, published by Asakura Shoten, pp.215-216, 1982, the fifth impression).
The ceramic disclosed in Japanese Patent Laid-Open No. 63-50365 is prepared by heat-pressure-forming a mixture of mica powder and a ZnO-SiO.sub.2 -B.sub.2 O.sub.3 -type ceramic. This ceramic excels in electrical insulation performance and machinability and has a low thermal expansivity, so that it is used as a useful heat-resistant insulating material. Its critical heat-resistance temperature, however, is as low as approximately 800.degree. C., so that its scope of use is inevitably limited. Supposedly, this is attributable to the fact that the melting point of the liquid phase substance generated in the ZnO-SiO.sub.2 -B.sub.2 O.sub.3 -type ceramic is around 800.degree. C.
In the ceramic disclosed in Japanese Patent Publication No. 64-345, a cordierite ceramic is used instead of the ZnO-SiO.sub.2 -B.sub.2 O.sub.3 -type ceramic disclosed in Japanese Patent Laid-Open No. 63-50365. According to the description in Japanese Patent Publication (No. 64-345), a mold is filled with a mixture of mica powder and cordierite powder to pressure-form the mixture into a molding, which is then baked by heating it up to its calcining temperature (1200.degree. to 1400.degree. C.), thereby preparing the ceramic desired. It can be assumed from experiences up to the present time that the calcined product obtained by this method has the following problem:
As the calcining temperature is higher than the decomposition point of mica (around 1100.degree. C.), it may be assumed that the mica suffers progressive thermal decomposition, with the result that the product machinability provided by the mica is deteriorated. Furthermore, the thermal decomposition is accompanied by the generation of gases such as fluorine. Thus, it can be assumed that the ceramic obtained is liable to be porous, which means it is not easy to prepare a compact product with this method.
The ceramic shown in the above-mentioned handbook, i.e., the one obtained by precipitating mica in glass, is the most excellent of the conventional ceramics known in terms of electrical insulation performance, precision machinability, etc. The mechanical strength it provides, however, is insufficient.